used the default settings, except that I changed the frame size to 720 by 480, as the video needs to run full screen.

Do you know what the Project's Preset was to begin with? You can probably get close, if the editor did things correctly, by going to the Project Panel, Rt-clicking on one of the Video Assets (files) and choosing Properties - about the middle of the list. That would give the original Image Size, plus other aspects. Make note, or do a screen-cap of the Properties dialogue screen.

Depending on what that reveals, we may need for you to make complete notes, or screen-caps of the Export dialogue box, to see all of the settings chosen.